FIG. 1 illustrates a prior art gas turbine of the type manufactured by the Assignee of the present invention for applications as an auxiliary power unit (APU) in airframes. The gas turbine 10 has a combustor 12 which combusts fuel supplied by a fuel system 14 to produce high pressure gas driving turbine rotor 16. A compressor 18 compresses gas which is applied to gas supply lines which extend between the exhaust stream, the start bypass valve 19 and the bleed value 19'. The rotor 16 also drives an alternator 20 and fuel pump 22 of the fuel system 14. Fuel lines are indicated by lines containing spaced circles. Igniter 22', which is activated by an exciter 24, is used in the starting of combustion within the combustor 12. Motor starter 26 is used for starting the turbine 10. The rotational speed of the turbine rotor 16 is sensed by speed sensor 28. Thermocouple 30 senses the exhaust gas temperature (EGT). An electronic sequence unit 32 controls the overall opening and closing of valves in the system with the section at a 6 o'clock position indicating a normally closed position. Memory 34 stores and displays the number of hours that the turbine has been operated and the number of times that the turbine has been started.
The fuel system 14 is comprised of a solenoid controlled start fuel valve 36; a solenoid controlled maximum fuel valve 39 for supplying extra fuel to the combustor to drive a maximum load and a solenoid controlled main fuel valve 40 which supplies fuel to the combustor during normal operation. The section of the valve 39 at the 12 o'clock position and the section of the valve 40 at the 6 o'clock position is normally closed. Each of the solenoid controlled fuel valves 36, 39 and 40 have either an on state in which fuel flow is unrestricted or an off state in which no fuel is passed by the solenoid controlled valve. The solenoid controlled valves 36, 39 and 40 do not proportionally meter the fuel flow with only an on maximum fuel flow and an off fuel flow being possible. In the prior art system of FIG. 1, the main fuel valve 40 during starting is not cyclically turned on and off and is not controlled as a function of temperature.
The turbine 10 utilized in an APU is required to start in extremely cold conditions which are present when an airframe is operating at high altitude and also in cold climates. In order to obtain reliable and fast starting, it is necessary to raise the combustion temperature within the combustor 12 during rotation of the turbine at a specified operational range typically between 10% and 50% of maximum shaft speed. Starting a gas turbine in extreme cold, such as at high altitude, is extremely difficult in the prior art with a significant degree of unreliability being present during each attempt to start the turbine. In extremely cold conditions, even though thermocouple 30 signals a rise in the exhaust gas temperature, there is not a corresponding acceleration of the rotor 16 of the turbine as sensed by the speed sensor 28. It is likely that combustion is occurring in the exhaust stream outside the combustor 12. This results in a situation where a malfunction in starting could arise. Furthermore, the electronic sequence unit 32 disables the flow of fuel to the combustor 12 when the sensed exhaust gas temperature sensed by thermocouple 30 rises above a predetermined maximum temperature, such as during hot starting conditions, which is a temperature at which damage to the combustor 12 and/or turbine 16 would occur by sustained operation. The prior art turbine starting sequences controlled by the electronic sequence unit 32 during hot starting conditions have the deficiencies of either having extremely high exhaust gas temperature with the concomitant damage to the combustor 12 and/or turbine wheel 16 or had high temperature shut downs.
FIGS. 2A and B illustrate a block diagram of the electronic sequence unit 32 of FIG. 1. Functional blocks are labelled. The electronic sequence unit 32 is microprocessor controlled by a program stored in memory.
U.S. Pat. Nos. 3,688,495, 4,015,426, 4,454,713 and 4,464,895 disclose the pulsing of fuel flow to a gas turbine.